In a damascene process, a stack of dielectric layers is first laid down on a semiconductor substrate that includes underlying layers of devices and interconnects, to form a structure that is eventually cut into multiple dies. The dielectric layers serve various functions, such as anti-reflection coating, insulating and etch-stopping. Grooves are then etched in the dielectric stack. The grooves are then filled with a conductive metal such as copper using a process such as plating. Finally, an exposed surface of the resulting structure is polished, to leave metal lines inlaid within the grooves.
There are several methods for measuring thickness, or change in thickness, of an upper-most layer in which the metal lines are formed. For example, in a method called “stylus profilometry” a stylus is run along the exposed surface and the height of the stylus is measured. This method requires contact to the surface of the structure and also requires that the substrate be precisely level prior to the damascene process. In addition, the stylus tips are fragile and require frequent replacement. As another example, focused ion beam scanning electron micrography (FIB-SEM) uses a focused ion beam to cut a hole in the structure. A scanning electron micrograph is then used to image the exposed cross-section. In a related method, the focused ion beam is used to cut out a section, which is then viewed with a transmission electron microscope (TEM). Such methods are slow, destructive, and not suited for monitoring the fabrication process.
Dielectric film thickness is also measured using ellipsometry. In one such method, light at multiple wavelengths is shone on a surface at an angle and the reflection is measured as function of incident polarization angle. Applicants note that a measurement made by this method uses a large spot size because a source of white light cannot be focused to the diffraction limit of a single wavelength. Applicants further note that ellipsometry does not provide the resolution required to measure profiles that vary across a distance (e.g. 20 μm) that is of the same order of magnitude as the spot size. Applicants also note that use of polarization as part of ellipsometry means polarization cannot be used to obtain measurements of dielectric properties within an array of metal lines.
U.S. Pat. No. 5,978,074 (which is incorporated by reference herein in its entirety) discloses an apparatus for characterizing multilayer samples. The apparatus focuses an intensity modulated pump beam onto the sample surface to periodically excite the sample, and also focuses a probe beam onto the sample surface within the periodically excited area. The power of the reflected probe beam is measured by a photodetector. The output of the photodetector is filtered and processed to derive the modulated optical reflectivity of the sample. Measurements are taken at a plurality of pump beam modulation frequencies. In addition, measurements are taken as the lateral separation between the pump and probe beam spots on the sample surface is varied. The measurements at multiple modulation frequencies and at different lateral beam spot spacings are used to help characterize complex multilayer samples. In the preferred embodiment, a spectrometer is also included to provide additional data for characterizing the sample.
Regarding use of a spectrometer, U.S. Pat. No. 5,978,074 states (at column 9, line 58 to column 10, line 10) “In the preferred embodiment, the subject apparatus further includes a spectrometer for providing additional data. As noted above, a white light source 120 is necessary for illuminating the sample for tracking on a TV monitor. This same light source can be used to provide spectral reflectivity data. As seen in FIG. 1, a beam splitter can be used to redirect a portion of the reflected white light to a spectrometer 142. The spectrometer can be of any type commonly known and used in the prior art. FIG. 4 illustrates one form of a spectrometer. As seen therein, the white light beam 122 strikes a curved grating 242 which functions to angularly spread the beam as a function of wavelength. A photodetector 244 is provided for measuring the beam. Detector 244 is typically a photodiode array with different wavelengths or colors falling on each element 246 in the array. The outputs of the diode array are sent to the processor for determining the reflectivity of the sample as a function of wavelength. This information can be used by the processor during the modeling steps to help further characterize the sample.”
Use of the white light for aligning the sample implies that the spectrometer is shown through the measurement objective lens. This is because the view for alignment must be the same as the view for measurement. Therefore, the spectrometer must be combined with the two laser measurement, adding complexity.
Applicants note that U.S. Pat. No. 5,978,074 is silent on how to “further characterize the sample,” other than to describe determining the sample's reflectivity as a function of wavelength as noted above. Applicants further note that U.S. Pat. No. 5,978,074 is also silent on what is done after a sample has been “further” characterized.
U.S. Pat. No. 5,978,074 also cites U.S. Pat. No. 5,074,669 granted to Opsal, which discloses using the combination of modulated optical reflectance plus the non-modulated reflectance of the two lasers to evaluate the implant dosage level in the semiconductor sample or to measure the thickness of a layer created by implantation.
According to an article entitled “Modules Are In, But Supertools Endure” by Alexander E. Braun in Semiconductor International November 1999 available on the Internet at www.semiconductor.net/semiconductor/issues/issues/1999/nov99/docs/imt.asp describes use of an ellipsometer in combination with a spectroscopic reflectometer. Specifically, the article states “The recently introduced Rudolph S200 system, for example, uses a proprietary multi-angle laser ellipsometer for thin films. The ellipsometer also is very sensitive to etch-to-zero applications, and yet tolerant of refractive index changes in underlying materials. The ellipsometric measurements also can be combined with a spectroscopic reflectometer and provide a better capability to measure overetch and characterize polish rate across the entire process window at more than 100 wafers per hour (five sites per wafer) . . . Rudolph has eliminated the primary cause of long-term difficulty in obtaining ellipsometer repeatability. If a microspot lens ellipsometer is used to measure on-product or a small spot, and high repeatability is attempted, stress birefringence in the lens—slight temperature changes over time—causes thickness measurements to vary by a few tenths of an Ångstrom. The new capability circumvents this, providing 0.01 Å repeatability.”